This invention is related to the problem of providing wear resistant, low-friction surfaces on components operating under high stress and frequently in corrosive conditions. A variety of means have been used in attempts to satisfy these requirements including: the hardening of steel surfaces by heat treatment, carburizing, nitriding, or ion implantation; the use of solid ceramic or cermet components; the application of coatings produced by thermal spray, chemical vapor deposition, physical deposition, electroplating (particularly with chromium); and other techniques. Depending on the application, all of these approaches have limitations. A particularly difficult application is that of high pressure gate valves that open or close at high velocity in the oil and gas production industry. Another application that is difficult to satisfy is the coating of aircraft landing gear components where, in addition to the problems of wear and friction, the fatigue characteristics of the substrate are of particular concern. It is the intent of this invention to provide thermal spray coatings that can satisfy these and a wide variety of the other problems.
Gate valves consist of a valve body which is located axially in piping or tubing through which the fluid to be controlled flows. Within the valve body is a "gate" which is a solid, usually metallic, rectilinear plate component with a circular hole through it. The gate slides between two "seats" which are circular annulus metallic, ceramic, or cermet components with an inside diameter approximately equal to the diameter of the hole in the gate. The seats are coaxially aligned with and directly or indirectly attached to the ends of the pipe or tubing within which the valve is located. When the hole in the gate is aligned with the holes in the seats, the fluid flows freely through the valve. When the hole in the gate is partially or completely misaligned with seats the fluid flow is impeded or interrupted; i.e., the valve is partially or fully closed. To avoid leakage of the fluid, it is essential that the surfaces in contact between the gate and the seats be very smooth and held tightly together. Valves may have springs or other devices within them to hold the seats firmly against the gate. When the valve is closed, the fluid pressure on the upstream side of the valve also presses the gate against the seat on downstream side.
Gate valves are usually operated by sliding the gate between the seats using an actuator attached to the gate with a rod or shaft called a "stem". Using a manual actuator results in a relatively slow gate movement, a hydraulic actuator results in a more rapid gate movement, and a pneumatic actuator usually results in a very rapid gate movement. The actuator must be able to exert enough force to overcome the static and dynamic friction forces between the seats and the gate. The friction force is a function of the valve design and the force of the fluid in the pipe when the valve is closed. This friction force can become extremely high when the fluid pressure becomes very high. Adhesive wear of the seats and/or the gate that can occur when the valve is opened and closed can also be a problem and become excessive under high-pressure conditions. An additional potential problem is that of corrosion. The oil and gas from many wells may contain very corrosive constituents. Thus, for many wells, the valves must be made of corrosion resistant materials, particularly the seats and gate where corrosion of the surfaces exacerbates the wear and friction problems.
For manually operated valves at low pressure, hardened steel seats and gates may be sufficient to combat the wear and friction problems. For pneumatic and hydraulic valves at higher pressures, thermally sprayed coatings, such as tungsten carbide or chromium carbide based coatings on both the gate and seat surfaces may be sufficient. Three of the best coatings of this type are the detonation gun coatings UCAR LW-15, a tungsten carbide-cobalt-chromium coating, UCAR LW-5, a tungsten carbide-nickel-chromium coating, and UCAR LC-1C, a chromium carbide + nickel-chromium coating. For some applications, the use of a solid cobalt base alloy, Stellite 3 or 6, for the seats with a hardened steel gate may be adequate. Other approaches have included laser or plasma transfer arc overlays of Stellite 6 and spray and fused alloys.
As wells became deeper, the pressures increased and the methods described above became inadequate. Two new coatings were developed that have become the benchmarks of the industry. One is UCAR LW-26, a tungsten carbide based coating, described more fully in U.S. Pat. No. 4,173,685. This coating is usually applied by plasma spray followed by a heat treatment. It has outstanding performance characteristics, but is relatively expensive to produce. The other is UCAR LW-45, a tungsten carbide-cobalt-chromium detonation gun coating with a unique microstructure which is able to perform well in most of the harsh conditions of present day oil and gas wells. However, as wells are drilled even deeper and the pressures became even higher, even these benchmark coatings can not satisfy the requirements for these extreme conditions, and there is no other solution available today.
Often coatings must be used for wear resistance on components that are very sensitive to fatigue. An example is the cylinder in an aircraft landing gear cylinder. Any coating that would crack under the tensile stresses imposed on the cylinder due to a bending moment during operation could propagate into the cylinder and cause a fatigue failure of the cylinder with disastrous results. The present coating on the cylinder is electroplated hard chromium, which has a negative effect on fatigue that must be compensated for with an excessively thick cylinder wall. The chromium plating runs against an aluminum-nickel-bronze bushing or bearing, so any replacement for the chromium plating must have good mating (adhesive wear) characteristics with this material as well. In addition, any coating must have good abrasion resistance in the event sand or other hard particles become trapped in the bearing. The presently used chromium electroplate is only marginally adequate. It should also be noted that electroplating of chromium has very undesirable environmental characteristics, and it would be advantageous to replace it in this and other applications. An alternative to the present system of a hard coating on the cylinder running against a relatively soft bushing or bearing surface would be to have both surfaces coated with a hard coating. This system would resist abrasion, but the coated surfaces must also have a low friction and be resistant to adhesive wear when running against each other.
The fatigue effects of a coating have often been related to the strain-to-fracture (STF) of the coating; i.e., the extent to which a coating can be stretched without cracking. STF has, in part, been related to the residual stress in a coating. Residual tensile stresses reduce the added external tensile stress that must be imposed on the coating to crack it, while residual compressive stresses increase the added tensile stress that must be imposed on the coating to crack it. Typically, the higher the STF of the coating, the less of a negative effect the coating will have on the fatigue characteristics of the substrate. This is true because a crack in a well-bonded coating may propagate into the substrate, initiating a fatigue crack and ultimately a fatigue failure. Unfortunately, most thermal spray coatings have very limited STF, even if they are made of pure metals which would normally be expected to be very ductile and easily plastically deform rather than crack.
Thermal spray coatings produced with low or moderate particle velocities during deposition typically have a residual tensile stress which can lead to cracking or spalling of the coating if it becomes excessive. Residual tensile stresses also usually lead to a reduction in the fatigue properties of the coated component by reducing the STF of the coating. Some coatings made with high particle velocities, particularly detonation gun and Super D-Gun coatings with very high particle velocities during deposition can have moderate to highly compressive residual stresses. This is especially true of tungsten carbide based coatings. High compressive stresses can beneficially affect the fatigue characteristics of the coated component. High compressive stresses can, however, lead to chipping of the coating when trying to coat sharp edges or similar geometric shapes. Thus it can be difficult to take advantage of the superior physical properties such as hardness, density, and wear resistance of the detonation gun and Super D-Gun coatings when coating such configurations.